The present invention relates to an apparatus for continuously measuring increases and decreases in moisture on ground and/or plant surfaces.
Dew is formed when the temperature falls below the dew point of the atmosphere. The resulting condensed water is then deposited on the surfaces of plants and on ground surfaces. During the heating period following sunrise, the majority of this liquid evaporates again. The quantity of dew formed and its period of dwell on a surface depend essentially on the temperature pattern, surface configuration and water absorption capability of the material constituting the surface. Dew measuring probes should therefore correspond in these characteristics to the natural surfaces and should have as good a contact as possible with the material on which the dew is deposited.
In the dew measuring art there are considered to be basically two types of instruments. The first type relates to instruments which measure the period of dwell of the dew; these instruments do not provide an indication of the quantity of dew, or only provide a very rough indication thereof.
This group of instruments includes the Wallin-Palhemus dew period recorder, the R-2777A precipitation detector, the Crossan and Lemmons-Clark dew recorder, the Stuzka-Uhler apparatus for measuring the moisture on leaves, the Theis-Calpouzos rain and dew recorder, the humectograph, and the Hearn surface moisture recorder.
The other type of instruments, for measuring the dew quantity, includes the Duvdevani dew gauge in which a piece of wood measuring 32.times.5.times.2.5 cm and covered with several layers of a red lacquer presents a surface tension with water such that characteristic drops form when moisture precipitates. The resulting drop configurations are compared with a set of 16 standard drop formers at known precipitation quantities of 0.01 to 0.35 mm. The piece of wood is fastened at sundown to a holder disposed about 1 m above ground. At sunrise the drop formation is evaluated.
There is also known the Potvin dew determination in which the dew is precipitated on a funnel-shaped collector surface and runs into a glass vessel with volume calibrations. The collector surface of this instrument is made of glass while Hungerford-Edgerton and Scott use various plastic materials.
There is also an Ekern weighing lysimeter which is a square box with sides of 150 cm in length and a height of 30 cm which is filled with soil and rests on one side on an abutment. The opposite side is supported by the hydraulic pressure of a water-filled pipe. Dew gain and evaporation losses are indicated by the level of the water in the pipe.
There is also a known dew integrator which includes a measuring element made of a folded cellulose adhesive tape on which the dew precipitates and which is inserted into a modified thermograph. The Hirt-MacDowell surface moisture recorder permits moisture to be deposited on a cylindrical surface of polystyrene. The weight of the cylinder is mechanically transmitted and recorded. In the Kessler-Fuess dew recorder, dew is collected on a slightly conical surface of blackened aluminum. Changes in weight are transmitted mechanically and recorded, while the Hiltner dew scale includes a circular nylon sieve which serves as the dew collecting surface and which is suspened from the balance arm of a scale. Movements of the scale balance are recorded on a writing drum.
A compliation of these dew measuring instruments can be found in the article by T. L. Noffsinger: "Survey of Techniques for Measuring Dew" in HUMIDITY AND MOISTURE, Volume II, edited by A. Wexler.
The first-mentioned type of instrument has the drawback that it merely registers the period during which the dew is present on the surface and does not permit measurements of the quantity of the dew.
Except for the Elkern lysimeter, all other known devices have the drawback that they inaccurately simulate the natural conditions for dew formation and evaporation on soil surfaces. Materials unlike the soil itself serve as collector surfaces and they are not in thermal contact with the soil surface. Some of them do not permit continuous recording. They also measure only the dew formation but not the evaporation.
Other of the known devices make only weight determinations. They have the drawback of being very sensitive to air movements so that the theoretically realizable measuring accuracy of about 0.01 mm precipitation cannot be obtained in practice. Dew formation on the scale balance and on the mount for the collector surfaces produces an additional error.
Only the elkern lysimeter uses natural soil as the collector surface. However, it has a mass of about 1000 kg. If, as has been asserted, 0.025 mm precipitation is measurable, a change in weight of about 60 g should be measurable.